This invention relates to dies for forming a sheet-like extrudate of fluent material.
Dies for forming sheet-like extrudates of fluent material are known and examples are found in the arts of forming films of thermoplastic or solvent-plasticized film and in the art of coating, for example the coating of layers of photographic materials onto a continuous substrate, such as a web of cellulose triacetate, polyethylene terephthalate or paper.
Such dies form a continuous flow of the fluent material along a duct into a sheet. In many applications it is very important that the sheet has uniform thickness not only along its length but also across its width. Much attention has been given to achieving widthwise thickness uniformity. FIGS. 1 and 2 of the accompanying drawings diagrammatically represent a known die 18 for extruding a sheet-like film of thermoplastic material. Only a few details of the die will now be described because the die is well known. The die body is formed of two die halves 20 and 22 secured together by means which are not shown. The die halves 20 and 22 contact one another at lands 24 and 26. The die is of the type known as a coat-hanger die and includes inner and outer cavities 28 and 30 respectively. Fluent material is supplied to the inner cavity 28 through a feed duct 32 located at the middle of the width of the die 18. The fluent material flows in both directions along the inner cavity, away from the duct 32. The fluent material which has spread throughout the inner cavity flows from the inner cavity 28 to the outer cavity 30 through a slot 34, sometimes termed the inner slot, which is formed between two spaced surfaces 36 and 38 of the two die halves 20, 22, respectively. The terms length, height and width will be used herein in relation to cavities and slots. It is to be understood that the term length is used in respect of dimensions parallel to the direction of flow of fluent material through the slots. The term height is used in relation to dimensions perpendicular to the parallel surfaces which bound the inner slot. The term width is used in respect of the dimension orthogonal to the length and height dimensions which are, of course, orthogonal to one another. The length of the slot 34 decreases with increasing distance from the location where the feed duct 32 opens into the cavity 28. The slot 34 discharges into the outer cavity 30. An outer slot 40 extends from the outer cavity 30 to the exterior of the die 18. The outer slot 40 is bounded by surfaces 42 and 44 of the die halves 20 and 22, respectively.
The outer cavity 30 is of uniform cross-sectional shape and area throughout its width dimension. The outer cavity 30 is formed by machining into only the die half 22 whereas the inner cavity 28 is formed by machining into both die halves 20 and 22, as may be seen in FIG. 2.
The outer slot 40 has uniform length throughout its width.
End plates 46 and 48 are secured to the ends of the die halves 20 and 22, as may be seen in FIG. 1. The end plates 46, 48 serve to close the ends of the inner and outer slots and the outer cavity and any small opening the inner cavity may have at its ends. With the tapering cross-sectional area of the inner cavity, associated with its known coat-hanger form, the cross-sectional area of the inner cavity decreases to zero or near zero at both of its ends, as may be seen at reference numerals 50 in FIG. 1. Thus, even when the end plates have been removed, the inner cavity is not accessible for cleaning and inspection.
As is known, the purpose of the coat-hanger form of the inner cavity 28 and of the tapered inner slot 34, is to give the sheet-like film which is extruded from the outer slot 40 uniform thickness across its entire width and yet maintain high wall shear stress throughout the width of the inner cavity and relatively low residence time and fast purging of fluent material. As is well known, the coat-hanger die has achieved a certain measure of success at rendering the thickness of the extruded film uniform but it has certain limitations which will now be considered.
Firstly, in that any one particular pair of inner cavity and inner slot is designed to optimize the thickness uniformity of the extruded film for one fluent material and one flow rate of that one fluent material or for other equivalent combinations of fluent material and flow rate, the die is dedicated to that one material and that one flow rate. Other materials, with other rheologies, and other flow rates will produce a film lacking widthwise thickness uniformity. This is a significant problem because it means that if near-perfect widthwise thickness uniformity is essential, a manufacturer must have available for use as many different dies as there are different combinations of materials and flow rates which it is desired to use in production, or the manufacturer must accept a degree of widthwise non-uniformity of thickness in order to avoid purchasing an excessive number of dies.
Secondly, the inner cavity is not accessible for cleaning and inspection merely by removing the end plates 46, 48. This is because the cross-sectional area of the inner cavity has tapered to zero or near zero at the ends of the die body. Thus, in order to clean and inspect the inner cavity 28, the two die halves have to be split apart, which, as is known to those skilled in the art, is a major operation because it entails, inter alia, during reassembly, securing the two die halves together in a manner which gives the outer slot 40 uniformity of height throughout its width.
It is an object of the present invention to overcome the above-described problems.